Measurement of wettability of the corneal epithelium. II. Contact angle method.

Adult rabbit corneas were saline-rinsed, mucolytic-rinsed, or wiped to remove mucus. The corneas were allowed to dry in air; sessile droplets of saline were placed on the surface at various drying times, and their equilibrium contact angle theta measured. Several stages of drying can be distinguished: 1) an initial period when excess water is present, with either complete spreading of droplets or very low value of theta; 2) a 'plateau' region lasting about 20-25 min in which theta is low and changes slowly with time, and reflects the intrinsic wettability of the surface; 3) a region when more severe drying takes place and theta rises sharply. Computed tables are used with known values of theta and the surface tension of the saline drops, to give the surface tension of the corneal surface and the interfacial tension between cornea and saline. Acetylcysteine-washed corneas showed a plateau value of about 16 degrees and corneal surface tension of 69.3 mN/m. Rinsed corneas remained fully wettable (theta = 0 degrees) for longer than 90 min; hence the plateau value must also be near zero and corneal surface tension about 72 mN/m. Mechanically-wiped corneas dried more rapidly and theta rose continuously with no marked plateau value, indicating much lower wettability due to cellular damage (about 40 mN/m).